Tingmei Wang

Morphology-Controllable Synthesis of Cobalt Oxalates and Their Conversion to Mesoporous Co3O4 Nanostructures for Application in Supercapacitors

In this work, one-dimensional and layered parallel folding of cobalt oxalate nanostructures have been selectively prepared by a one-step, template-free, water-controlled precipitation approach by simply altering the solvents used at ambient temperature and pressure. Encouragingly, the feeding order of solutions played an extraordinary role in the synthesis of nanorods and nanowires. After calcination in air, the as-prepared cobalt oxalate nanostructures were converted to mesoporous Co(3)O(4) nanostructures while their original frame structures were well maintained. The phase composition, morphology, and structure of the as-obtained products were studied in detail. Electrochemical properties of the Co(3)O(4) electrodes were carried out using cyclic voltammetry (CV) and galvanostatic charge-discharge measurements by a three-electrode system. The electrochemical experiments revealed that the layered parallel folding structure of mesoporous Co(3)O(4) exhibited higher capacitance compared to that of the nanorods and nanowires. A maximum specific capacitance of 202.5 F g (-1) has been obtained in 2 M KOH aqueous electrolyte at a current density of 1 A g(-1) with a voltage window from 0 to 0.40 V. Furthermore, the specific capacitance decay after 1000 continuous charge-discharge cycles was negligible, revealing the excellent stability of the electrode. These characteristics indicate that the mesoporous Co(3)O(4) nanostructures are promising electrode materials for supercapacitors.

Preparation and tribological properties of graphene oxide/nitrile rubber nanocomposites

Graphene oxide (GO)/nitrile rubber (NBR) nanocomposites with various contents of GO were prepared by a solution-mixing method,in this study. The GO sheets were exfoliated from natural fake graphite by an improved Hummers method and could be further dispersed homogeneously in NBR matrix. The thickness and size of the GO sheets were observed by atomic force microscopy and transmission electron microscopy. The tribological properties of the GO/NBR nanocomposites were evaluated on a ring-block MRH-3 wear tester under dry sliding and water-lubricated conditions. The worn surface morphologies of the GO/NBR nanocomposites were observed by a scanning electron microscopy. It was found that under dry sliding, both the friction coefficient (COF) and specific wear rate of the nanocomposites decreased dramatically at first, then increased with increasing GO contents, while under water-lubricated condition, both the COF and specific wear rate of the nanocomposites decreased with increasing GO contents. Finally, the friction and wear mechanisms of the GO/NBR nanocomposites were tentatively proposed.

Controlled synthesis of mesoporous hematite nanostructures and their application as electrochemical capacitor electrodes

In this work, iron oxalate (FeC₂O₄·2H₂O) with different morphologies was synthesized through a simple solution-based direct precipitation process. Three samples with distinct morphologies, i.e., microrods with a parallelogram-like cross-section, nanorods, and multi-layered nanosheets, could be obtained in a selective manner. We found that the shapes of the iron oxalate could be controlled just through simply altering the solvents used. The one-dimensional (1D) characteristic of the infinite linear chains and the selective interaction between solvents and various crystallographic planes of FeC₂O₄·2H₂O played an important role in the formation of FeC₂O₄·2H₂O with different morphologies. Phase-pure hematite (α-Fe₂O₃) had be obtained by annealing these as-prepared FeC₂O₄·2H₂O precursors without significant alterations in morphology. The as-obtained mesoporous α-Fe₂O₃ products had high specific surface areas with narrow pore size distribution. The electrochemical properties of the α-Fe₂O₃ electrodes were investigated using cyclic voltammetry (CV) and galvanostatic charge-discharge measurements by a three electrode system. The electrochemical experiments revealed that they showed a structure-dependence in their specific capacitances. The mesoporous multi-layered nanosheets exhibited a significant structurally induced enhancement of capacity properties associated with their novel structure characteristic in addition to the high specific surface area. They can present the highest specific capacitance value (116.25 F g⁻¹) and excellent long cycle life within the voltage window from - 0.6 to 0 V. This method can be easily controlled and is expected to be extended to produce other functional materials with controlled structure.

Controlled growth of pyrite FeS2 crystallites by a facile surfactant-assisted solvothermal method

A simple surfactant-assisted ethylene glycol-mediated solvothermal method is reported for the synthesis of pyrite crystallites with cubic and octahedral shapes for the first time. The process involved the reduction of sulfur and the reaction between S2− and Fe2+ in the presence of surfactant. By changing the concentration of NaOH and properly adjusting some other experimental parameters, pyrite crystallites with a defined crystalline structure have been realized. It was demonstrated that the different surfactants can adsorb on certain faces selectively, the subsequent competing growth between [100] and [111] directions directed the final shape of the particles. NaOH was found to be crucial for the formation of the pyrite cubes and octahedral crystallites by not only acting as the pH regulator but also determining the reduction ability of ethylene glycol. The influences of solvothermal temperature, the dosage of sulfur, the concentration of NaOH and the kinds of the surfactants on the shape of the crystallites were investigated systematically. The magnetic properties were also studied. Moreover, the method demonstrated here could be of great potential for controlling the synthesis of other metal chalcogenides with controlled shapes and structures.

Well-defined core-shell carbon black/polypyrrole nanocomposites for electrochemical energy storage.

The well-defined core-shell carbon black/polypyrrole (CB/PPy) nanocomposites were prepared via the in situ chemical oxidative polymerization of pyrrole from the surfaces of the carbon black (CB) nanoparticles, with poly(2-hydroxy-3-(methacryloyloxy) propane-1-sulfonate) (PHMAS) as both the surfactant and the dopant. The nanocomposites exhibited the high conductivity at room temperature and the weakly temperature dependence of conductivity from 283 to 423 K. When the core-shell CB/PPy nanocomposites were used as the electrode materials for the supercapacitors, the maximum discharge capacity of 366 F/g was achieved, after being corrected for the weight percentage of the PPy phase at the current density of 5 mA/cm(2) in 1.0 M NaNO(3) electrolyte solution.

High-strain shape memory polymer networks crosslinked by SiO2

We addressed a novel high-strain shape memory polymer network constructed from SiO2 microspheres core functionalized with polycaprolactone arms. Using abundant surface hydroxyl of silica, epsilon-caprolactone was initiated and uniform SiO2@PCL macroparticles were formed; after cross-linking of SiO2@PCL by MDI, a high-strain shape memory polymer network of SiO2–SMP was obtained. Compared to traditional SMP having similar composition, the SiO2–SMP exhibits excellent mechanical strength and good shape memory properties due to the introduction of SiO2 as a cross-linking point. The elastic moduli (E) and tensile strength (σm) of the SiO2–SMP were improved to 500 MPa and 90 MPa, respectively, at room temperature. It could be attributed to the enhancement of the combining force among polymer chains caused by the crosslinking networks, and the rigidity of the polymer owing to the addition of SiO2 as filler. High shape fixing ratios (∼100%) and recovery ratios (more than 95%) were also exhibited for SiO2–SMP from the second circle. The comparatively low recovery ratios were exhibited in the first circle; the obvious evidence of the existence of the irreversible bond breakage is attributed to it. Furthermore, the mechanical and shape memory properties could be adjusted by changing the size of silica microspheres and the molecular weight of the PCL.

A tough shape memory polymer with triple-shape memory and two-way shape memory properties

In this study, a tough shape memory polymer network based on polydopamine, poly(e-caprolactone) and diisocyanate was synthesized in three steps. Fourier transform infrared spectroscopy was used to confirm the synthesis process. The tensile tests demonstrated the good mechanical properties of the materials with a tensile modulus and tensile strength reaching 362 and 43 MPa, respectively, at room temperature. The thermal properties of the polymer networks were investigated using differential scanning calorimetry and dynamic mechanical analysis. With two broad transition temperatures, the dual-shape memory properties were greatly affected by the deformation temperature, which was investigated in detail. Moreover, the polymers also showed good triple-shape memory and two-way shape memory effects.

High performance shape memory polyimides based on π–π interactions

A series of polyimides (PIs) with different chain structures were synthesized by a two-step method. The influences of chain conformations on their thermal and mechanical properties were investigated by dynamic mechanical analysis, thermogravimetric analysis, as well as by a universal testing machine. All PIs exhibited high glass transition temperature (>240 °C) and thermal decomposition temperature (>480 °C). Moreover, all these PIs could show shape memory properties more or less, especially ODA–BPDA with a shape recovery ratio (Rr) greater than 93%. The high value of Rr was mainly due to the existence of π–π interaction, a type of non-covalent interaction. The dependence of Rr on π–π interaction was investigated in detail by UV-vis spectroscopy. Finally, atomic oxygen (AO) exposure experiments showed that the shape memory properties of ODA–BPDA were affected little by the erosion due to AO.

Stimuli-responsive composite particles as solid-stabilizers for effective oil harvesting.

The polymer-grafted magnetic composite particles have been synthesized and developed to harvest oil by use of their speical wettability. Different from gravity-driven oil-water separation, the prepared polymer brushes-grafted magnetic composite particles can act as solid-stabilizers that diffuse to the oil-water interfical region and effectively minimize the direct oil-water interfical area by volume exclusion, whereas the magnetic Fe3O4 core allows easy separation of Pickering emulsions from oil-water mixture under an external magnetic field. When the emulsions were heated from room temperature to 50 °C, the coil-to-globule transition of poly(N-isopropylacrylamide) (PNIPAM) acts as the driving force for the destabilization of the emulsion, thereby achieving the release of oil. The novel materials can be used in aspects of oil-water separation, inducing oil droplet transport and release of lipophilic substrates.

Shape controlled growth of pyrite FeS2 crystallites via a polymer-assisted hydrothermal route

Well-defined pyrite FeS2 microcubes and micro-octahedra with high-yield and good uniformity were successfully synthesized by a convenient and simple polymer-assisted hydrothermal method. Employing S as sulfur source and FeCl2·4H2O as the precursor in the presence of polyvinylpyrrolidone (PVP) and polyvinyl alcohol (PVA), cube-shaped and octahedron-shaped pyrite FeS2 crystallites were successfully obtained by simply adjusting the NaOH concentration, and the size of the product can be readily tuned by varying the reaction parameters. The effects of various NaOH concentrations and the polymer dosage on the morphology evolution process were investigated systematically. The phase composition, morphology, structure, and properties of the as-obtained products were studied by X-ray diffraction, field emission scanning electron microscopy, (high-resolution) transmission electron microscopy, Fourier transform-infrared spectroscopy, X-ray photoelectron spectrometry, and UV-Vis spectrometry. It was found that besides the capping effect of the polymer reagents, the “synergistic effect” or “cooperative effect” between the PVA and the PVP also played a crucial role in directing the growth of the FeS2 crystallites. The possible formation mechanism of the FeS2 phase and morphology was tentatively proposed based on a series of contrast experiments. Moreover, the optical properties were also preliminarily studied. This method can be easily controlled and is expected to be extendable to the fabrication of other metal chalcogenides with controlled shape and structure.